Process for the electrolytic production of metals of the alkalis or alkaline earths



Oct. 11, 1932. w. SIECKE 1,882,525

PROCESS FOR THE ELECTROLYTIC PRODUCTION "F METALS OF THE ALKALIS ORALKALINE EARTHS Filed Nov. 15, 1929 aka (M4436.

Patented Oct. 11 '1932 UNITED STATES PATENT OFFICE WOLFHART srnoxn, or,IRANKFORT-ON-THE-MAIN, GERMANY PROCESS FOR THE ELECTROLYTIC PRODUCTIONOF METALS OF THE ALKALIS OR ALKALINE EAB'IHS I application filedNovember 13, 1929, Serial No. 407,003, and in Austria November 18, 1928.

This invention relates to the production of alloys of the alkaline earthmetals or mixtures of the same, by the electrolysis of the correspondinghalides with the aid of molten cathodes, especially lead cathodes Theknown processes, which are especially intended for the production ofalloys of lead and alkaline earth metals, are attended with the drawbackthat, aside from other difficulties, they require an extremely largeconsumption of current.

I have found that these difiiculties and drawbacks can be overcome invarious ways.

In the course of the electrolysis of molten halides of alkaline earthmetals, such as alkaline earth chlorides or saline mixturescontainingthe'same, in contact'with a cathode of molten lead the liberated metalsform alloys with the molten lead, such as, for example, the compound Pbca, which cover the surface of the lead bath as a solid crust whichprevents thelead from taking up any further amounts of the alkalineearth metal, for example calcium, deposited at the cathode so that thecalcium which has not been taken up rises to the surface of theelectrolyte and burns there or forms calcium carbide with the anodewhich consi"ts of carbon or graphite. To prevent this inconvenience, ithas constituent, it has been found necessary, in

general, to maintain the bath at temperatures i above 630 (3., andpreferably above 650 (1.,

for example between 650 and 700 C.

\Vhen treating a mixture of, for example, calcium chloride and sodiumchloride or potassium chloride, with a fused lead cathode,

mixing of the molten cathode metal, for

it was observed that there was formed during the progress of theelectrolysis, the compound Pb Ca, the melting point of which lies atabout 630 G. Since this compound segregates out directly both in thesolid as well as in the fused state, the danger arises when working attemperatures below 650 C., that the surface of the lead cathode may onoccasion solidify to a solid crust, so that the calcium separating outcan no longer be taken up by the lead, but rises to the surface where itburns, or it reacts with the carbon to form calcium carbide. The currentyield then decreases; and the impurities set up an anode effect. Thesedrawbacks are avoided in accordance with the invention by keeping thefused cathode at suitably high temperatures.

I have moreover foundit necessary to carry out the process whilethoroughly mixing the fused cathode in order to avoid disturbingseparations such as for example the segrega tion of Pb ca even in thefused state when working at sufliciently high temperatures, for exampleabove 650 C. I have found that the example the molten lead, can beeffected in a particularly favorable manner by the use of verticallyreciprocating stirring members The stirrer may for example consist of aperforated sheet of metal which is moved up and down in thefusedcathode. This method of mixing-ofi'ers the advantage that the sur faceof the fused cathode metal remains horizontal, whereas if it is agitatedor if other types of stirring means are used, disturb- 85 ances mayarise, for example, the centrifugal action of rotating stirrers causesthe surfaces of the molten cathode-metal to assume a fun-' nel-likeshape, the electrolysis being thereby 9o adversely affected and, inparticular, the danger of short circuits with the anode arises.

The accompanying drawing is adiagram- 'matic representation of one formof appara-v tus suitable for carrying out my invention.

In the drawing, 1 is the containerffmade of metal, for example iron, 2is the electrolyte,

3 is the molten lead cathode and 4 and 5 are anode plates. The agitatingmember 6 is provided with apertures 6' and is given a re- 10ociprocating vertical motion in the molten cathode bymeans of shaft 7.

Furthermore, mixtures of halides, of the alkaline-earth metals withother halides of a metal-other than alkaline earth metal which are adatedto prevent the saline mixture from ta ing up and, retaining water atthe temperature of the electrolysis such as for example, mixtures ofchlorides of alkaline earth metals with halides, forexample chloridesofsodium or potassium, for example in an amount of at least 20% of thetotal saline mixture, have been found specially suitable for theelectro'deposition of alkaline earth metals, such as for example,calcium.-

The water introduced with the electrolyte exertsa disturbing actionrintwo-ways; on the one hand because the yield per unit of current isreduced by reason of the accompanying electrolysis of the water; and onthe,

other hand because any light metal formed or already present reacts withthe water to form reaction products, such as CaOwhich promote theoccurrence of the anode effect. :It.

has been found that the dehydrating action of added substances such aschlorides of .alkali metals, upon the halides of the alkaline earthmetals possessing a great aflinity for water, such as for example,calcium chloride generally increases as the amount of the addedsubstances increases. Relatively large amounts of sodiu however, areseparated out in addition to he alkaline earth metal, such as calcium,from mixtures of halogen salts,

containing comparatively large amounts. of sodium in addition to calciumsalts, for example. This is not the case when employing potassiumchloride even in considerable amounts. The employment for elec- Jtrolysis of such mixtures rich in potassium chloride, which for example,may contain about 20 to of calcium chloride and about 35 to of.potassium chloride, not only enables the salt serving as the fitstcharge for the bath to be dehydrated with ease,'but above all, enablesthe halides consumed during the electrolysis, which may on occasion behalides which can only with difiiculty-be dehydrated, such as calciumchloride, to be introduced into the electrolytic bath in awater-containing form, for example, in the form of ordinary calciumchloride containingnabout 7 to 10% of water,whilst maintaining thetemperatures, of for example, upwards of 630? C. At the increasedtemperatures of the each piece of the salt introduced is immedi atelysurrounded after the manner of Leidenfrost drops with a pellicle ofsteam and is therefore prevented from coming directly into contact withthe melt until the completion of the rapid evaporation of the water,where-. upon the now completely dehydrated salt is readily taken up bythe melt. In this manner, it is possible, by employing the abovementioned higher temperatures, to avoid the very disagreeable explosivephenomena which are otherwise unavoidable if attempts are made tointroduce incompletely dehydrated salts, such as for example, thecalcium chloride hereinbefore set forth, at less elevated temperaturesinto the melt, since in this case the entire amount of the waterpresviolenc By virtue of the possibility, even when employing halideshaving an extraordinary aflinity for water, such as calcium chloride otbeing able to avoid a preliminary del1y-' dration of the amounts ofthese salts to be introduced during the electrolysis a substantialcheapening and simplification of the process is possible, inasmuchas itis now no longer necessary to completely dehydrate these salts with alarge expenditure of fuel and crucible material and only then tointroout evaporates all at once with explosive duce them into theelectrolysis. Moreover, 1

there is also the further advantage that the saline mixture required forthe initial charging of the electrolytic cell can be made in a much moresimple manner than if the constituents have first to be dehydratedseparately or if saline mixtures are employed which could not be aseasily dehydrated.

Similar advantages as are-obtainable according to the foregoing, in theproduction of calcium by the addition of potassium chloride, preferablyin relatively large amounts, can naturally also be obtained in theelectrolysis of other metals of 'the alkaline earths by the employmentof deliquescent halides of the metals to be deposited in combinationwith other halides of the same or of another metal which are adapted toform with the salt in question, mixtures which are easily dehydratableand which can be easily kept free from water in a fused state or whichform mixtures which are not adapted to take up water on thesubsequentaddition of the water-contain- "ing-salt tobe electrolyzed. The.deposition at the molten cathode of cations of the salt or salt mixture,employed as addition, besides the principal metal, may be avoided whenthe process is to be carried out under definite operating conditions,such as temperature, voltage and so forth, by so choosing the salt orsalt mixture, or when the salt or salt mixture to be'added is fixed, byso choosing the remaining operating conditions that the undesiredcathodic depositions are avoided.

For practicalworking it has been found advisable" to heat the bathexternally as well as by the electric current. Although it is I an easymatter to heat the salt melt to 700 C. while continually stirring themolten cathand over, since in electrical heating the electrolyte acts asresistance, it has been'found in practice that the underlying moltencathode metal, which offers practically no resistance to'the current,may remain up to 200 C. cooler than the supernatent heated salt layer.For the reasons given above it is, however, important to maintain thecathode metal, for example lead, at the temperature therein specifiedduring the electrolysis. To this end use is advantageously made ofelectrolyzing vessels, the lower portions of which are freelyaccessible, in order that the cathode metal contained therein can beexternally heated, while the upper portions are insulated, for examplewith fire-clay. It is essential that this fire clay layer should extenddownwards through the saline melt and as far as the surface of the lead,in order to assure complete electrical insulation between the anode andthe cathode. That is to say, the metallic calcium must be depositedinthe lead exclusively and not on any uninsulated iron parts above thelead.

It has also been found that, in carrying out the electrolytic process,the so-ealled anode effect, i. e., the occurrence of thin insulatinglayers which are broken through by the electric current with formationof small arcs, is a notable, source of disturbance. It has now beenascertained that the anode effeet is promoted by high voltage, while, onthe other hand, this inconvenience can be counteracted by operating withcomparatively low current densities. In general it has been foundadvantageous to employ current densities which do not substantiallyexceed 4 amps. per sq. cm. and which are preferably below that figure.

The invention is applicable to the treatment of halides of the alkalineearth metals, such as salts of calcium, magnesium and the like, andparticularly to mixtures of various originating materials. In additionto chlorides, it can be applied to the treatment of fluorides, forexample, and also to mixtures of chlorides and fluorides, such as, forexample, calcium fluoride and sodium chloride. The lead may be replacedby other metals or me tallic alloys, which are molten at thetemperatures coming under consideration, to serve as cathode material,such as aluminium and the like.

- cathode having a temperature above 600 C.

ode. I

2. Process of preparing alloys of lead and alkaline earth metals whichcomprises electrolyzing a molten salt mixture containing at least onehalide of the alkaline earth metal to be deposited and at least oneother halide of a metal other than an alkaline earth metal in contactwith a cathode of molten lead the temperature of which is above 600 C.while continually stirring the molten cathode.

3. Process of preparing alloys of lead and alkaline earth metals whichcomprises electrolyzing a molten salt mixturecontaining at least onehalide of the alkaline earth metal to be deposited and at least oneother halide of a metal other than an alkaline earth metal in contactwith a cathode of molten lead the temperature of which is between 630and 700 while continually stirring the molten cathodes l. Process ofpreparing alloys-oflead and alkaline earth metals which compriseselectrolyzing a molten salt mixture containing at least one halide ofthe alkaline earth metal to be deposited and at least one other halideof a metal otherthan an alkaline earth metal in contact with a cathodeof molten lead the temperature of which is at least 630 C. whilecontinually stirring the molten cathode without disturbing the smoothupper surface thereof to any great extent.

5. Process ofpre'paring alloys of laid and alkaline earth metals whichcomprises electrolyzing a molten salt mixture containing at least onehalide of the alkaline earth metal to be deposited and at least oneother halide of metal other than an alkaline earth metal in contact witha cathode of molten lead the temperature of which is at least 630 C.while continually stirring the molten cathode in a vertical direction.v

6. Process of preparing alloys of alkaline earth metals which compriseselectrolyzing a molten salt mixture containing at least one halide ofthe alkaline earth metal to be deposited and a suificient amount of atleast one other salt, which is adapted to prevent the salt mixture fromtaking up and retaining water at the temperature of electrolysis, incontact with a cathode of molten lead the temperature of which is atleast 630 C. while continually stirring the molten cathode.

7. Process of preparing alloys of alkaline earth metals which compriseselectrolyzing a molten salt mixture containing at least one halide ofthe alkaline earth metal to be deposited and at least onehalide of analkali metal in contact with a cathode of molten,

halide of calcium and at least one halide of j an alkali metal incontact with a cathode molten lead, the temperature of which is at least630 C. while continually stirring the molten cathode.

'9. Process of preparing alloys of alkaline earth metals which compriseselectrolyzing' a molten salt mixture containing at least one halide ofthe alkaline earth metal to be deposited and sodium chloride in contactwith a cathode of molten lead the temperature of which is at least 630C. while continually molten salt mixture containing calcium chloride andpotassium chloride in an amount of at least 35% of the total salinemixture in contact with a cathode of molten lead the temperatureof whichis at least 630C. wglile continually stirring the molten cath- I o e.

12. Process of preparing alloys of lead lead the temperature of which isat least 630 v C. while continually stirring the molten cathode andheating the cathode externally.

16. Process of preparing alloys of lead with calcium whichcompriseselectrolyzing a molten salt mixture containing calcium chloride and atleast one chloride of an alkali metal in contact with a cathode ofmolten lead the temperature of which is at least 630 C. whilecontinually stirringthe molten cathode the current density employed notsub-- stantially exceeding 4 .amperes centimeter of anode surface.

17 Process of preparing alloys of alkaline per square I earth metalswhich compnses electrolyzing' a molten salt mixture containing at leastone halide ofthe alkaline earth metal to be .deposited and sodiumchloride in an amount of at least 20% of the total -saline mixture incontact with a cathode of molten lead the temperature of which is atleast 630 C. while continually stirring the molten cathode.

" In testimony whereof, I affix m signature.

WOLFHART IEGKE- with calcium which comprises ele'ctrolyzing a moltensalt mixture containing about 35 to of potassium chloride and about 65to 20% of calcium chloride in contact with a cathode of molten lead thetemperature of which is at least 630 C. while continually stirring themolten cathode.

13. Process of preparing alloys of lead and alkaline earth metals whichcomprises elec-' trolyzing a molten salt mixture containing at least onehalide of the alkaline earth metal to be deposited and at least oneother halide of a metal'other than an alkaline earth metal in contactwith a cathode of molten. lead the temperature of which is at least 630Cwhile continually stirring the molten cathode and H replacing thealkaline earth halide consumed during electrolysis in a water-containingform.

14. Process of preparing alloys of lead with calcium which compriseselectrolyzing a molten salt mixture containin calcium chloride arid atleast one chloride 0 an alkali metal'iri'contact with a cathode ofmolten lead 4 thetemperature of which is at least 630 C..

while continually stirring the molten cathode and adding portions ofcalcium chloride in a water-containing form to replace the calcium vchloride consumed during the electrolysis. 15. Process of preparingalloys of lead with calcium which comprises electrolyzingr a molten saltmixture containing calcium chloride and at least one chloride of analkali metal in contact with a cathode of molten

